EP4335563A1 - Skeleton member - Google Patents

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Publication number
EP4335563A1
EP4335563A1 EP22798898.7A EP22798898A EP4335563A1 EP 4335563 A1 EP4335563 A1 EP 4335563A1 EP 22798898 A EP22798898 A EP 22798898A EP 4335563 A1 EP4335563 A1 EP 4335563A1
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EP
European Patent Office
Prior art keywords
frame member
section
closed cross
cross
hardness
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP22798898.7A
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German (de)
English (en)
French (fr)
Inventor
Takahiro Aitoh
Yuri Toda
Ryo Tabata
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Nippon Steel Corp
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Nippon Steel Corp
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Publication date
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Publication of EP4335563A1 publication Critical patent/EP4335563A1/en
Pending legal-status Critical Current

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B62—LAND VEHICLES FOR TRAVELLING OTHERWISE THAN ON RAILS
    • B62D—MOTOR VEHICLES; TRAILERS
    • B62D21/00—Understructures, i.e. chassis frame on which a vehicle body may be mounted
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D53/00—Making other particular articles
    • B21D53/88—Making other particular articles other parts for vehicles, e.g. cowlings, mudguards
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B62—LAND VEHICLES FOR TRAVELLING OTHERWISE THAN ON RAILS
    • B62D—MOTOR VEHICLES; TRAILERS
    • B62D25/00—Superstructure or monocoque structure sub-units; Parts or details thereof not otherwise provided for
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B62—LAND VEHICLES FOR TRAVELLING OTHERWISE THAN ON RAILS
    • B62D—MOTOR VEHICLES; TRAILERS
    • B62D29/00—Superstructures, understructures, or sub-units thereof, characterised by the material thereof
    • B62D29/007—Superstructures, understructures, or sub-units thereof, characterised by the material thereof predominantly of special steel or specially treated steel, e.g. stainless steel or locally surface hardened steel
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D17/00—Forming single grooves in sheet metal or tubular or hollow articles

Definitions

  • the present invention relates to a frame member having excellent energy absorption efficiency.
  • a hollow member obtained by processing a steel sheet into a predetermined closed cross-sectional shape has been used as a frame member of a vehicle.
  • Such a frame member is required to realize the weight reduction and to exhibit a sufficient proof stress and energy absorption performance in a case where a bending load is applied thereto due to a collision.
  • Examples of the method primarily used for realizing the weight reduction include a method of reducing the weight by thinning a member by an increase in the proof stress and energy absorption performance due to an increase of the strength of a steel sheet. Therefore, in recent years, a steel sheet capable of exhibiting a tensile strength greater than 1.8 GPa may be used as a material of a frame member.
  • Patent Document 1 discloses a collision-proof reinforcing member for a vehicle which is formed of a formed thin sheet to increase buckling resistance, including at least a main body portion and a pair of side wall portions integrated with the main body portion via folding portions, in which the main body portion is provided with a recessed bead extending in a center of the main body portion in a width direction along a longitudinal direction of the main body portion, and the recessed bead is provided so that an effective width c' as a distance between the recessed bead and the folding portion satisfies a specific range.
  • Patent Document 1 Japanese Unexamined Patent Application, First Publication No. 2009-286351
  • the bead is provided in consideration of the effective width, and thus it is possible to suppress elastic buckling and to improve the proof stress.
  • it is required to take measures for further increasing the energy absorption efficiency which is the amount of energy absorbed per unit cross-section area of the frame member.
  • the present invention has been made in view of the above problems, and an object of the present invention is to provide a frame member having excellent energy absorption efficiency.
  • the width of the wall portion of the recessed bead part and the hardness standard deviation ratio are controlled within appropriate ranges, it is possible to prevent bending fracture while suppressing elastic buckling. Accordingly, a high degree of energy absorption performance can be obtained even in a case where a high-strength thin member is used. Accordingly, it is possible to exhibit excellent energy absorption efficiency.
  • the present inventors intensively studied the configuration of a frame member capable of exhibiting excellent energy absorption efficiency.
  • the frame member has a bending proof stress of a certain level or higher.
  • a bead along a longitudinal direction of the member, it is possible to improve the bending proof stress in a case where an input load in a bending direction is applied due to a collision.
  • elastic buckling deflection
  • a required bending proof stress may not be obtained, and excellent energy absorption efficiency may not be exhibited.
  • the frame member realizes deformation in a desired deformation mode immediately after a bending load is applied thereto due to a collision, in order to efficiently absorb impact energy.
  • excellent energy absorption efficiency may not be exhibited.
  • the present inventors paid attention to the fact that the above-described problems hinder a further increase of the strength and thinning of a high strength steel sheet.
  • the present inventors further conducted studies, and found that in a case where the width of a wall portion of a recessed bead and the hardness standard deviation ratio are controlled within appropriate ranges, it is possible to prevent bending fracture while suppressing elastic buckling.
  • the present inventors found that thanks to such control, it is possible to solve the above-described problems which may occur in using a high strength steel sheet, and to exhibit excellent energy absorption efficiency, and completed the present invention.
  • the "longitudinal direction” means a member axis direction of a frame member, that is, a direction in which the axis extends.
  • the "bending compressive assuming surface” means a part in the frame member, where it is assumed that a compressive stress in the longitudinal direction is generated in a case where the frame member receives a bending load due to a collision or the like.
  • the "flat part” means a linear part in a cross section perpendicular to the longitudinal direction of the frame member, specifically, a part having a radius of curvature larger than a maximum external dimension of the cross section.
  • the maximum external dimension means the maximum straight line length between end portions at two arbitrary points in the cross section.
  • the "recessed bead part” means a part protruding from the bending compressive assuming surface toward the inside of a closed cross section portion in a cross section perpendicular to the longitudinal direction of the frame member.
  • the "corner part” means a non-linear part excluding the flat part and the recessed bead part in the cross section perpendicular to the longitudinal direction of the frame member.
  • the "width” means a line length along the circumferential direction of the closed cross section portion, and for example, the “width of the wall portion” means a line length between one end and the other end of the wall portion.
  • the "effective width” is an effective width W e obtained from Formula (1) based on Karman's effective width theory, that is, Karman's effective width formula.
  • W e t 4 ⁇ 2 E 12 1 ⁇ ⁇ 2 ⁇ y
  • the effective width W e 577 t/ ⁇ h.
  • the effective width W e can be obtained from the above expression.
  • the "effective width ratio” is a ratio of a width H 0 of the wall portion of the recessed bead part to the effective width W e , and is a value calculated by H 0 /W e . It can be said that the smaller the value of the effective width ratio, the more hardly the elastic buckling in the wall portion occurs in the cross-sectional shape.
  • the "surface layer portion” means a region between: a depth position where a distance from a surface of the steel sheet to the depth position separated therefrom in the sheet thickness direction is 1% of the sheet thickness of the steel sheet; and a depth position where a distance from the surface of the steel sheet to the depth position separated therefrom in the sheet thickness direction is 5% of the sheet thickness of the steel sheet.
  • the "thickness middle portion” means a depth position where a distance from the surface of the steel sheet to the depth position separated therefrom in the sheet thickness direction of the steel sheet is 3/8 of the sheet thickness.
  • the "surface of the steel sheet" set as the reference of the depth position means a surface of a base steel sheet.
  • the surface of the steel sheet in a state where the plating, painting, and rust have been removed is set as the reference of the depth position.
  • a surface layer coating such as plating, painting, rust, or the like is formed on the surface of the base steel sheet, the boundary between the surface layer coating and the surface of the base steel sheet is easily identified by various known methods.
  • the “amount of energy absorbed” is an amount of energy absorbed calculated from the relationship between the impactor reaction force (load) and the stroke when a rigid flat impactor is allowed to collide with the bending compressive assuming surface in a state in which both ends of the frame member are completely restricted.
  • the "energy absorption efficiency" is an amount of energy absorbed per cross-section area of the frame member.
  • the energy absorption efficiency is an amount of energy absorbed per cross-section area in a closed cross section where the cross-section area is minimum in a closed cross section perpendicular to the longitudinal direction of the member.
  • FIG. 1 is a perspective view of the frame member 1.
  • FIG. 2 is a cross-sectional view along the cutting-plane line A1-A1 of FIG. 1 , and is a cross-sectional view perpendicular to the longitudinal direction of the frame member 1.
  • FIG. 3 is an enlarged view of a region surrounded by A of FIG. 2 .
  • the frame member 1 is formed of a main body 10 having a hollow tube shape extending in the longitudinal direction. That is, the frame member 1 is a member in which a cross section perpendicular to the longitudinal direction is a closed cross section as a single unit.
  • the bending compressive assuming surface is disposed to face the outside of a vehicle body, whereby in a case where a collision is applied, a load capacity is exhibited against the compressive stress in the bending compressive assuming surface.
  • the bending compressive assuming surface is provided with a recessed bead part 100 interposed between two first flat parts 11, 11.
  • the load capacity may not be fully exhibited due to the deflection of the flat part in a case where a compressive load is applied.
  • the recessed bead part 100 is disposed to be interposed between the two first flat parts 11, 11 as in the frame member 1, the load capacity improvement effect can be obtained.
  • First corner parts C1, C1 are formed at outer end portions of the first flat parts 11, 11.
  • Two second flat parts 13, 13 whose surfaces face each other extend from end portions of the first corner parts C1, C1 on the sides opposite to the first flat parts 11, 11.
  • second corner parts C2, C2 bent in directions approaching each other are formed at end portions of the second flat parts 13, 13 on the sides opposite to the first corner parts C1, C1. End portions of the second corner parts C2, C2 on the sides opposite to the second flat parts 13 are connected to each other by a third flat part 15.
  • the recessed bead part 100, the first flat parts 11, 11, the second flat parts 13, 13, the third flat part 15, the first corner parts C1, C1, and the second corner parts C2, C2 form a closed cross section portion.
  • the recessed bead part 100 includes first bent portions 121, 121, wall portions 123, 123, second bent portions 125, 125, and a bottom portion 127.
  • the first bent portions 121, 121 are parts which are bent toward the inside of the closed cross section from end portions of the two first flat parts 11, 11 facing each other. Since a part having a radius of curvature of 50 mm or greater is regarded as a part of the wall portion, the radius of curvature of the first bent portion 121 is less than 50 mm.
  • the radius of curvature of the first bent portion 121 may be, for example, 3 mm to 5 mm.
  • the wall portions 123, 123 are parts protruding toward the inside of the closed cross section portion via the first bent portions 121 and 121.
  • the wall portions 123, 123 are linear parts having a radius of curvature of 50 mm or greater.
  • the second bent portions 125, 125 are parts which are bent in directions facing each other from end portions of the wall portions 123, 123 on the sides opposite to the first bent portions 121, 121. Since a part having a radius of curvature of 50 mm or greater is regarded as a part of the wall portion 123 or a part of the bottom portion 127, the radius of curvature of the second bent portion 125 is less than 50 mm.
  • the radius of curvature of the second bent portion 125 may be, for example, 3 mm to 5 mm.
  • the bottom portion 127 is a part linearly connecting end portions of the second bent portions 125, 125 on the sides opposite to the wall portions 123, 123 to each other.
  • the width H 0 of the wall portion 123 of the recessed bead part 100 is set to be 2.5 times or less the effective width W e .
  • the width H 0 of the wall portion 123 is set to be 0.5 times or greater the effective width W e .
  • the upper limit of the effective width W e is preferably 60 mm or less in order to obtain a required proof stress.
  • the sheet thickness of the recessed bead part 100 is preferably 1.2 mm or less from the viewpoint of weight reduction.
  • the sheet thickness of the recessed bead part 100 is less than 0.4 mm, elastic buckling is likely to occur in the wall portion 123 of the recessed bead part 100, and thus the setting range of the width H 0 is greatly restricted. Accordingly, the sheet thickness of the recessed bead part 100 is preferably 0.4 mm or greater.
  • the frame member 1 is formed by: heating a steel sheet for hot stamping up to an austenite range; forming, in a state in which the steel sheet is held in a predetermined temperature range, the steel sheet into a predetermined shape through hot stamping for performing pressing while performing a quenching treatment by a press die and punch having a rapid cooling mechanism; and then joining end surfaces together.
  • the frame member 1 formed as described above has a strength greater than 1.8 GPa in terms of tensile strength.
  • the Vickers hardness of the thickness middle portion of the wall portion 123 of the recessed bead part 100 in the frame member 1 is 520 Hv or greater in a hardness test performed by the method described in JIS Z 2244: 2009 with a test load of 300 gf (2.9 N).
  • the hardness of the thickness middle portion in the wall portion 123 of the recessed bead part 100 is set to 520 Hv or greater in terms of Vickers hardness.
  • the upper limit of the hardness of the thickness middle portion is not particularly specified, but may be 850 Hv or less in terms of Vickers hardness.
  • a method of measuring the hardness of the thickness middle portion in the wall portion 123 of the recessed bead part 100 is as follows.
  • a sample having a cross section perpendicular to the sheet surface is collected from the wall portion 123 of the recessed bead part 100.
  • the cross section is prepared as a measurement surface, and the measurement surface is subjected to a hardness test.
  • the size of the measurement surface depends on the measuring apparatus, but may be about 10 mm ⁇ 10 mm.
  • the method of preparing the measurement surface is performed according to JIS Z 2244: 2009.
  • the measurement surface is polished using silicon carbide paper ranging from #600 to #1500, the measurement surface is mirror-finished using a liquid obtained by dispersing a diamond powder having a particle size of 1 ⁇ m to 6 ⁇ m in a dilution liquid such as alcohol or pure water.
  • a dilution liquid such as alcohol or pure water.
  • the measurement surface mirror-finished as described above is subjected to the hardness test by the method described in JIS Z 2244: 2009.
  • Hardness is measured using a micro-Vickers hardness tester at 30 points that are arranged at intervals of three times or more the indentation under a load of 300 gf in the position at a depth of 3/8 of the sheet thickness of the sample, and the average value of the measured values is defined as the hardness of the thickness middle portion.
  • the bending performance is increased by appropriately controlling the ratio between the standard deviation of hardness frequency distribution in the thickness middle portion and the standard deviation of hardness frequency distribution in the surface layer portion in the wall portion 123 of the recessed bead part 100.
  • the frame member 1 according to the present embodiment even in a case where a high-strength material is applied, fracture during deformation is suppressed, and it is possible to exhibit significantly excellent energy absorption efficiency compared to the related art.
  • the hardness standard deviation ratio which is a value obtained by dividing the standard deviation of hardness frequency distribution in the surface layer portion by the standard deviation of hardness frequency distribution in the thickness middle portion (depth position of 3/8 of the sheet thickness) is controlled to be less than 1.0.
  • the present inventors have found through experiments that in a case where the hardness standard deviation ratio is less than 1.0 in applying a hot-stamped material having a strength greater than 1.8 GPa, the maximum bending angle in a VDA bending test based on the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry can be considerably improved.
  • FIG. 4 is a graph showing the results of the VDA bending test using a steel sheet of a 2.0 GPa-grade material with a thickness of 1.4 mm. It is found that the less the hardness standard deviation ratio is than 1.0, the larger the maximum bending angle (°) in the VDA bending test and the higher the bending angle ratio. That is, in a case where the hardness standard deviation ratio is less than 1.0, fracture hardly occurs during deformation due to a load in an axial direction, and excellent energy absorption efficiency can be exhibited.
  • the standard deviation ratio is more preferably less than 0.8.
  • the hardness frequency distribution in the thickness middle portion and the hardness frequency distribution in the surface layer portion are acquired by a Vickers hardness test.
  • a sample having a cross section perpendicular to the sheet surface is cut out from an optional position including the wall portion 123 of the recessed bead part 100.
  • the cross section is prepared as a measurement surface, and the measurement surface is subjected to a hardness test.
  • the size of the measurement surface depends on the measuring apparatus, but may be about 10 mm ⁇ 10 mm.
  • the method of preparing the measurement surface is performed according to JIS Z 2244: 2009.
  • the measurement surface is polished using silicon carbide paper ranging from #600 to #1500, the measurement surface is mirror-finished using a liquid obtained by dispersing a diamond powder having a particle size of 1 ⁇ m to 6 ⁇ m in a dilution liquid such as alcohol or pure water.
  • a dilution liquid such as alcohol or pure water.
  • the measurement surface mirror-finished as described above is subjected to the hardness test by the method described in JIS Z 2244: 2009.
  • the hardness in the surface layer portion is measured using a micro-Vickers hardness tester.
  • Hardness is measured at 30 points that are arranged at intervals of three times or more the indentation under a load of 300 gf, and the hardness frequency distribution in the surface layer portion is obtained.
  • the thickness middle portion depth position of 3/8 of the sheet thickness
  • hardness is measured at 30 points that are arranged at intervals of three times or more the indentation under a load of 300 gf, and the hardness frequency distribution in the thickness middle portion is obtained.
  • a known statistical method is used to obtain the standard deviations of the hardness frequency distribution in the surface layer portion and the hardness frequency distribution in the thickness middle portion, obtained as a result of the Vickers hardness test described above.
  • the hardness frequency distribution in the surface layer portion is the same as the hardness frequency distribution in the thickness middle portion, and the hardness standard deviation ratio is 1.0.
  • the hardness standard deviation ratio becomes a value different from 1.0.
  • the frame member 1 formed of a steel sheet for hot stamping in a case where the metallographic structure in only the surface layer portion and the vicinity thereof is modified, the distribution and unevenness of the hardness in the surface layer portion are suppressed, and the hardness standard deviation ratio between the surface layer portion and the thickness middle portion can be made less than 1.0.
  • the hardness standard deviation ratio can be controlled by adjusting a highest heating temperature and a holding time in decarburization annealing of the steel sheet for hot stamping, which is a known technology.
  • the decarburization annealing temperature maximum attainment temperature of the steel sheet
  • the residence time in a temperature range of 700°C to 950°C is 5 seconds to 1,200 seconds under a moist atmosphere containing hydrogen, nitrogen, or oxygen.
  • the hardness standard deviation ratio can be made less than 0.8.
  • At least one surface layer portion of the wall portion 123 may satisfy the above hardness standard deviation ratio condition. However, it is preferable that the surface layer portions on both sides of the wall portion 123 satisfy the above hardness standard deviation ratio condition.
  • elastic buckling is suppressed by controlling the width H 0 of the wall portion 123 of the recessed bead part 100, and fracture during deformation can be suppressed by controlling the hardness standard deviation ratio.
  • the energy absorption efficiency can be significantly improved while the thickness middle portion of the wall portion 123 of the recessed bead part 100 has sufficient hardness of 520 Hv or greater in terms of Vickers hardness.
  • the frame member 2 according to the second embodiment is different from the frame member 1 according to the first embodiment in that a cross section perpendicular to a longitudinal direction is formed as a closed cross section by two members. That is, in the frame member 2, a closed cross section portion is configured by two members joined to each other.
  • FIG. 5 is a perspective view of the frame member 2.
  • FIG. 6 is a cross-sectional view along the cutting-plane line A2-A2 of FIG. 5 , and is a cross-sectional view perpendicular to the longitudinal direction of the frame member 2.
  • FIG. 7 is an enlarged view of a region surrounded by B of FIG. 6 .
  • the frame member 2 has a closed cross section portion formed by joining a first frame member 20 to a second frame member 30. That is, the closed cross section portion is configured to include the first frame member 20 and the second frame member 30.
  • the first frame member 20 is a member having a hat-shaped cross section, and its top sheet surface functions as a bending compressive assuming surface.
  • a recessed bead part 200 is interposed between two first flat parts 21, 21.
  • First corner parts C1, C1 are formed at outer end portions of the first flat parts 21, 21.
  • Two second flat parts 23, 23 whose surfaces face each other extend from end portions of the first corner parts C1, C1 on the sides opposite to the first flat parts 21, 21.
  • second corner parts C2, C2 bent in directions away from each other are formed at end portions of the second flat parts 23, 23 on the sides opposite to the first corner parts C1, C1.
  • Third flat parts 25, 25 extend in directions away from each other from end portions of the second corner parts C2, C2 on the sides opposite to the second flat parts 23.
  • the second frame member 30 is a flat plate-shaped steel sheet having: a pair of joint parts 31, 31 which are in surface contact with the third flat parts 25, 25 of the first frame member 20 and are joined by spot welding or the like; and a flat part 33 interposed between the pair of joint parts 31, 31.
  • the recessed bead part 200, the first flat parts 21, 21, the second flat parts 23, 23, the first corner parts C1, C1, and the second corner parts C2, C2 in the first frame member 20, and the flat part 33 in the second frame member 30 form a closed cross section portion.
  • the recessed bead part 200 includes first bent portions 221, 221, wall portions 223, 223, second bent portions 225, 225, and a bottom portion 227.
  • the configuration of the recessed bead part 200 is the same as that of the recessed bead part 100 described in the first embodiment, detailed description thereof will be omitted.
  • the closed cross section portion may be formed of two or more members joined to each other.
  • the frame member 1 according to the first embodiment has a configuration in which one recessed bead part is provided on the bending compressive assuming surface, but two or more recessed bead parts may be formed on the bending compressive assuming surface. That is, two recessed bead parts 100A, 100A may be formed on the bending compressive assuming surface as in a frame member 1A according to a first modification example shown in FIG. 8 .
  • the number of flat parts is not particularly limited, and there may be at least two flat parts connected to a bent portion of the recessed bead part.
  • the above-described recessed bead part 100 has the pair of wall portions 123, 123 extending and facing each other, but may have a pair of wall portions 123B, 123B extending and inclining relative to each other as in a recessed bead part 100B according to a second modification example shown in FIG. 9 .
  • the recessed bead 100B includes first bent portions 121B, 121B bent toward the inside of a closed cross section, the wall portions 123B, 123B protruding and inclining relative to each other toward the inside of a closed cross section portion via the first bent portions 121B, 121B, second bent portions 125B, 125B bent in directions facing each other from end portions of the wall portions 123B, 123B on the sides opposite to the first bent portions 121B, 121B, and a bottom portion 127B linearly connecting end portions of the second bent portions 125B, 125B on the sides opposite to the wall portions 123B, 123B to each other.
  • the above-described recessed bead part 100 has the pair of second bent portions 125, 125 and the bottom portion 127, but may have an aspect in which a pair of wall portions 123C, 123C extending and inclining relative to each other are connected to each other by a single second bent portion 125C as in a recessed bead part 100C according to a third modification example shown in FIG. 10 .
  • the recessed bead 100C includes first bent portions 121C, 121C bent toward the inside of a closed cross section, wall portions 123C, 123C protruding and inclining relative to each other toward the inside of a closed cross section portion via the first bent portions 121C, 121C, and the second bent portion 125C connecting end portions of the wall portions 123C, 123C on the sides opposite to the first bent portions 121C, 121C to each other. That is, the recessed bead 100C is not configured to include the linear bottom portion 27 shown in the first embodiment.
  • the frame members 1, 2 have a uniform cross-sectional shape over the whole length, but may not have a uniform cross-sectional shape over the whole length, and the above-described closed cross section portion may be present in a part of the whole length in the longitudinal direction.
  • the closed cross section portion is present in preferably 50% or greater, and more preferably 80% or greater of the whole length in the longitudinal direction.
  • the frame members 1, 2 are applied to members to which a compression input is to be applied mainly in the axial direction at the time of the collision, among structural members of a vehicle body.
  • FIG. 11 is a view showing a vehicle frame 300 as an example to which the frame members 1, 2 are applied.
  • the frame members 1, 2 can be applied to a frontside member 301, a rearside member 303, a side sill 305, an A pillar 307, a B pillar 309, a roof rail 311, a floor cross 313, a roof cross 315, and an under reinforcement 317 among structural members of a vehicle body.
  • a steel sheet A and a steel sheet B having a sheet thickness of 0.5 mm were prepared.
  • Both the steel sheet A and the steel sheet B are steel sheets for hot stamping to be subjected to hot stamping.
  • the decarburization annealing temperature (maximum attainment temperature of the steel sheet) was set to 750°C, and the residence time in a temperature range of 700°C to 750°C was set to 300 seconds under a moist atmosphere provided by mixing hydrogen and nitrogen, to modify the metallographic structure in only a surface layer portion and the vicinity thereof.
  • FIG. 12 is a schematic view for explaining a cross-sectional shape of a rectangular tube member in examples. As shown in FIG. 12 , a substantially square cross section where the length of one side was 74 mm was designed as a basic design in all experimental examples.
  • the radii of curvature of four corner parts C were all designed to be 5 mm, and the radii of curvature of bent portions in the recessed bead part were all set to 3 mm.
  • Table 1 shows material characteristics in the flat parts of the rectangular tube members after hot stamping.
  • the metallographic structure was the same in a thickness middle portion and a surface layer portion, and thus the hardness standard deviation ratio in the flat part was 1.0. That is, in Experimental Examples 2A, 3A, 4A, 5A, 6A, and 7A in which the recessed bead part was provided, the hardness standard deviation ratio in the wall portion of the recessed bead part was 1.0.
  • the hardness standard deviation ratio in the flat part was 0.65 since the metallographic structure of a thickness middle portion was not modified, but the metallographic structure of a surface layer portion was modified. That is, in Experimental Examples 2B, 3B, 4B, 5B, 6B, and 7B in which the recessed bead part was provided, the hardness standard deviation ratio in the wall portion of the recessed bead part was 0.65.
  • a rigid flat impactor was allowed to collide with the bending compressive assuming surface of each of the rectangular tube members at a speed of 80 km/h in a state in which both ends in the longitudinal direction were completely restricted.
  • the deformation states at the time of the collision, the states in which fracture occurred, and the absorbed energy calculated from the impactor reaction force (load) and the stroke were compared.
  • Table 2 shows the setting conditions and the results for each experimental example. Table 2] Experiment No. Steel Sheet Used Width H 0 of Wall Portion (mm) Effective Width Ratio Cross Section Area (mm 2 ) State of Fracture Absorbed Energy (J) Energy Absorption Efficiency (J/mm 2 ) Remarks No. 1A A - - 144 Crack Penetration 120 0.84 Comparative Example No. 1B B - - 144 No Cracks 166 1.16 Comparative Example No. 2A A 0 0 147 Crack Penetration 162 1.10 Comparative Example No. 2B B 0 0 147 No Cracks 189 1.28 Comparative Example No. 3A A 5 0.5 152 Crack Penetration 202 1.33 Comparative Example No.
  • Experimental Example 1B is a comparative example in which the hardness standard deviation ratio is appropriately controlled, but the recessed bead part is not formed.
  • the proof stress improvement effect due to the recessed bead part provided was not obtained, and the energy absorption efficiency was inferior.
  • Experimental Example 2B is a comparative example in which although the hardness standard deviation ratio is appropriately controlled and the recessed bead part is formed, the effective width ratio is low.
  • the absorbed energy was low, and the energy absorption efficiency was inferior.
  • FIG. 13 is a graph for comparison of the energy absorption efficiency relative to the effective width ratio based on the experimental results shown in Table 2. As shown in this graph, it is found that in a case where a bead shape in which the effective width ratio is in an appropriate range is formed and the hardness standard deviation ratio is appropriately controlled, the energy absorption efficiency is significantly improved.
  • Second Example experiments for verifying that excellent energy absorption efficiency could be exhibited by providing a plurality of recessed bead parts were performed by using the same steel sheet A and steel sheet B as those in First Example.
  • the radii of curvature of four corner parts C were all designed to be 5 mm, and the radii of curvature of the recessed bead parts were all set to 3 mm.
  • a rigid flat impactor was allowed to collide with the bending compressive assuming surface of each of the rectangular tube members at a speed of 80 km/h in a state in which both ends in the longitudinal direction were completely restricted.
  • the deformation states at the time of the collision, the states in which fracture occurred, and the absorbed energy calculated from the impactor reaction force (load) and the stroke were compared.
  • Table 3 shows the setting conditions and the results for each experimental example.
  • Table 3 Experiment No. Steel Sheet Used Width H 0 of Wall Portion (mm) Effective Width Ratio Cross Section Area (mm 2 ) State of Fracture Absorbed Energy (J) Energy Absorption Efficiency (J/mm 2 ) Remarks No. 8A A 0 0 150 Crack Penetration 220 1.47 Comparative Example No. 8B B 0 0 150 No Cracks 272 1.81 Comparative Example No. 9A A 7 0.7 175 Crack Penetration 404 2.31 Comparative Example No. 9B B 7 0.7 175 No Cracks 500 2.86 Invention Example
  • Experimental Example 8B is a comparative example in which although the hardness standard deviation ratio is appropriately controlled, the effective width ratio is 0. In this comparative example, the absorbed energy was low due to early buckling, and the energy absorption efficiency was inferior.

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  • Transportation (AREA)
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EP22798898.7A 2021-05-06 2022-04-26 Skeleton member Pending EP4335563A1 (en)

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JP5119477B2 (ja) 2008-05-30 2013-01-16 新日鐵住金株式会社 耐座屈性に優れた車両用耐衝突補強材及びその製造方法
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